The present invention generally relates to electrochromic devices and more particularly relates to control systems for electrochromic devices.
Electrochromic devices include electrochromic materials that change their optical properties in response to the application of an electric current or an electric potential. Electrochromic devices generally include a series of thin films that are deposited directly onto the surface of a transparent substrate, such as a conventional window made of glass or plastic. Electrochromic devices are typically incorporated into architectural and transportation window applications, display windows and cabinets, or any application requiring a changeable tinted glass. By selectively controlling the transmission of optical energy through these windows, particularly windows on buildings and vehicles, tremendous cost savings in terms of heating and cooling can be realized.
While a great deal of prior art exists to control electrochromic devices, very little of it is useful for controlling electrochromic devices with substantial electronic leakage current, including devices made with solid-state ion conductors. In addition, previous systems for electrically heating electrochromic devices require modifications to the easy to manufacture two-bus bar electrode arrangement. Finally, in situations where safe operating voltage depends on temperature, integrating temperature-measurement solutions can be difficult.
Most electrochromic devices that have been developed to date use polymeric electrolyte layers. Unfortunately, there are a number of disadvantages associated with polymeric devices. For example, polymeric devices have a significant weight and thickness and require two pieces of transparent substrate (e.g. glass) to be laminated together. In addition, the durability of the polymer approach for glazing applications is suspect due to the instability of polymer electrolyte layers when exposed to UV light and temperature.
It has recently been determined that the above problems can be avoided by replacing the polymeric electrolyte layers of electrochromic devices with solid state inorganic electrolyte layers. Electrochromic devices having solid state inorganic electrolytes are lighter and thinner than polymeric devices, and are more durable when exposed to UV light and temperature. However, the solid state inorganic electrolyte layers allow electronic leakage current through the electrolyte. This electronic leakage phenomenon may cause slower switching speed, longer switching times, and a slow loss of charge state when the device is being held in a steady state, which could lead to spontaneous coloration or bleaching depending on the electrochemical potentials within the device.
There have been a number of security devices incorporated into windows. U.S. Pat. No. 4,972,176 to Vallance discloses a polymeric security window with an integrated intrusion detector. The security window includes a laminated pair of polymeric panels having electrically conductive rods sandwiched therebetween. Glass fibers, having electrically conductive coatings, extend from edge to edge of the plastic window panels and are distributed in the rods, which substantially cover the entire window area. The index of refraction of the coated glass fibers is similar to that of the surrounding plastics, making the electrically conductive rods substantially invisible. The conductive rods are connected in series, and a current source provides a constant current through the circuit. The resistance of the circuit increases when a rod is severed, or punctured. A change in the magnitude of the voltage required to maintain constant current in the circuit indicates that a window panel and at least some of the glass fibers have been damaged.
U.S. Pat. No. 5,648,758 to Tweadey, II et al. discloses a pre-assembled glass breakage detector having a carrier substrate designed to carry an electrical circuit subassembly. The electrical circuit subassembly includes an electrically conductive fragile film and spaced electrical connectors. A method of making a glazing unit security system includes pre-assembling one or more such glass breakage detector appliqués and then applying the electrical subassembly thereof to the surface of a glazing pane, and connecting electrical needs from continuity lost detection circuitry to the electrical connectors. In a motor vehicle application, the appliqués can be applied to stationary and/or moveable vehicle windows, being especially advantageous for use on a hidden surface area of moveably mounted glazing panes. The fragile film loses electrical continuity upon disruption of the underlying glazing pane, when the pane is broken.
U.S. Pat. No. 5,627,509 to Gajewski et al. discloses a glazing unit security system having an electrically conductive strip of polymeric material adhered to the surface of a tempered glass pane. A security monitoring element electrically connected to the conductive strip senses and responds to a loss of electrical continuity of the conductive strip. The conductive strip is non-self-integral, such that it would not survive with electrically continuity a fracture of the underlying glass panel. Since a fracture of the tempered glass panel will result in overall fracturing of the panel, it would cause loss of electrical continuity of the conductive strip. The security system further includes an alarm or other device responsive to a loss of electrical continuity of the conductive strip. When installed in a motor vehicle, the security system may include disabling means to prevent normal operation of a motor vehicle in response to loss of electrical continuity of a conductive strip.
In spite of the above advances, there remains a need for a controller for an electrochromic device that incorporates security monitoring capabilities at little or no additional cost. There is also a need for a controller for an electrochromic device that can measure the temperature of the EC device without using additional components or external measuring instruments. There is also a need for a controller for an electrochromic device that is capable of operating the device at safe operating voltage levels, which may depend on the measured temperature. There is also a need for a controller for an electrochromic device that compensates for electronic leakage current, which may occur in devices having solid state ion conductors. There is also a need for a controller for an electrochromic device that enables the device to be heated without requiring an extra or distinct heating element. There also remains a need for a control system for an electrochromic device that is capable of monitoring the transmission state of the device and effectively controlling the transmission level of the device.